Charged particle beam alignment method and charged particle beam apparatus

ABSTRACT

An object of the present invention is to provide a charged particle beam apparatus and an alignment method of the charged particle beam apparatus, which make it possible to align an optical axis of a charged particle beam easily even when a state of the charged particle beam changes. The present invention comprises calculation means for calculating a deflection amount of an alignment deflector which performs an axis alignment for an objective lens, a plurality of calculation methods for calculating the deflection amount is memorized in the calculation means, and a selection means for selecting at least one of the calculation methods is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a charged particle beam apparatus, moreparticularly to a charged particle beam apparatus which corrects anoptical axis deviation of a charged particle optical system and issuitable for obtaining a high resolution image.

2. Discussion of the Background

A charged particle beam apparatus typified by a scanning electronmicroscope acquires desired information, for example, a sample image,from a sample by scanning a finely converged charged particle beamthereonto. Because, in such a charged particle beam apparatus, lensaberration occurs and a resolution of the sample image decreases with anoptical axis deviation relative to a lens, a high precision axisalignment is required to acquire a high resolution sample image.Therefore, excitation current of an objective lens and the like wereperiodically changed in a conventional axis alignment, and operationalconditions of a deflector (aligner) for the axis alignment was manuallyadjusted so as to minimize a motion thereof at this time. As atechnology for performing the axis alignment automatically, there is onedisclosed in Japanese Patent Laid-Open No. 2000-195453. According todescriptions of this gazette, disclosed is the technology for changingan excitation set value of an alignment coil based on a transition of anelectron beam radiation position which changes between two excitationconditions of the objective lens. Moreover, in Japanese Patent Laid-OpenNo. 2000-331637, a technology is disclosed, which performs a focuscorrection based on a detection of a position deviation between twoelectron microscope image obtained under different optical conditions.

If the lens deviates from a center of an astigmatism corrector forperforming an astigmatism correction of a charged particle beam, a fieldof view moves in aligning astigmatism, and the alignment of theastigmatism becomes difficult. Therefore, another aligner (deflector)which controls the position of the charged particle on the sample inassociation with an operation of the astigmatism corrector is provided,and a motion of an image relative to a change of a set value of theastigmatism corrector is cancelled, thus performing a correction of thefield of view so that an observed image does not move in aligning theastigmatism. At this time, though a signal in proportion to the setvalue of the astigmatism corrector is input to the aligner forcorrecting the field of view, a proportional coefficient must bedetermined so as to cancel the motion of the image in aligning theastigmatism. To perform this alignment, an operation, in which the setvalue such as current and the like of the astigmatism corrector wasperiodically changed and a proportional coefficient to minimize a motionof an image at this time is determined, was carried out.

SUMMARY OF THE INVENTION

To perform the alignment of the optical axis manually as describedabove, a technique supported by experience is necessary, so that analignment precision varies depending on an operator and a time isrequired for the alignment in some cases. Moreover, with respect to theabove described automatic alignment of the optical axis, parameter forthe alignment changing depending on optical conditions must be memorizedfor each optical condition. When an observation is tried by changing theoptical condition, a register operation is required for each opticalcondition. Even if the parameters for the alignment were used under thesame optical condition, there is a problem that an alignment based onthe registered parameters is difficult because of a change of theoptical axis with the passage of time. Furthermore, there is apossibility that the operator performs an observation based on a sampleimage deteriorated without noticing that the optical axis deviates.

An object of the present invention is to provide a method of aligning acharged particle beam and a charged particle beam apparatus, which makeit possible to align an optical axis easily even when an opticalcondition is changed or even when a state of a charged particle beamchanges by a change of the optical axis with the passage of time.

Another object of the present invention is to provide a charged particlebeam apparatus suitable for enabling an optical axis to be automaticallyaligned.

To achieve the foregoing objects, the present invention provides amethod of aligning a charged particle beam and a charged particle beamapparatus, in which when a deflection condition of an alignmentdeflector is set to a first state in aligning an axis of a chargedparticle beam relative to an optical device influencing the chargedparticle beam with an alignment deflector, the optical device is changedinto at least two states, and a first deviation between first and secondsample images obtained at this time is detected, and when the deflectioncondition of the alignment deflector is set to a second state, theoptical device is changed into at least two states, a second deviationbetween third and fourth sample images obtained at this time isdetected, and an operation condition of the alignment deflector isdetermined based on information concerning the first and seconddeviations.

According to the constitution described above, an axis alignment with ahigh precision is possible irrespective of an optical condition of thecharged particle beam.

To achieve another object of the present invention, provided is acharged particle beam apparatus which comprises a charged particlesource, an optical device for aligning a charged particle beam emittedfrom the charged particle source, and an alignment deflector forperforming an axis alignment for the optical device, the chargedparticle beam apparatus further comprising calculation means forcalculating a deflection amount of the alignment deflector, wherein aplurality of calculation methods for calculating the deflection amountare memorized in the calculation means, and selection means forselecting the calculation methods is provided.

According to the constitution described above, it is possible to executethe axis alignment with a high precision automatically irrespective ofan optical condition of the charged particle beam. Note that anotherconstitution of the present invention will be described in describingembodiments of the invention in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an outline of a constitution of ascanning electron microscope as an example of the present invention.

FIG. 2 is an outline of a processing flow for correcting an axisdeviation relative to an objective lens.

FIG. 3 shows a principle for correcting the axis deviation relative tothe objective lens.

FIG. 4 is an outline of a processing flow for correcting an axisdeviation relative to an astigmatism corrector.

FIG. 5 is a drawing illustrating an example of a message when the axisdeviation is detected.

FIG. 6 is a drawing illustrating an example of an axis deviationdetection processing to which an image quality decision processing isadded.

FIG. 7 is a drawing illustrating a setting screen for settingenvironments of an automatic axis deviation correction.

FIGS. 8( a) and 8(b) are drawings illustrating a displaying example of acorrection amount graph.

FIG. 9 is an outline of a processing flow for correcting an axisdeviation.

FIG. 10 is a processing flow for detecting an image deviation.

FIG. 11 is an outline of a processing flow at the time when an automaticoperation is executed.

FIG. 12 is a drawing illustrating a setting screen for settingenvironments of an automatic axis deviation correction at the time whenthe automatic operation is executed.

FIG. 13 is an outline of a processing flow of an automatic astigmatismalignment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described with reference tothe accompanying drawings below.

FIG. 1 is an outline of a constitution of a scanning electron microscopeas an example of the present invention. A voltage is applied between anegative electrode 1 and a first positive electrode 2 by a high voltagecontrol power source 20 controlled by a computer 40, and a primaryelectron beam 4 is drawn out from the negative electrode 1 with apredetermined emission current. An acceleration voltage is appliedbetween the negative electrode 1 and a second positive electrode 3 bythe high voltage control power source 20 controlled by the computer 40,and the primary electron beam 4 emitted from the negative electrode 1 isaccelerated. The primary electron beam 4 is accelerated to advance to alens system at a rear stage. The primary electron beam 4 is converged bya convergence lens 5 controlled by a lens control power source 21, andan unnecessary region of the primary electron beam 4 is removed by adiaphragm plate 8. Thereafter, the primary electron beam 4 is convergedby a convergence lens 6 controlled by a lens control power source 22 andan objective lens 7 controlled by an objective lens control power source23 onto a sample 10 as a minute spot. The objective lens 7 can adoptvarious styles such as an in-lens style, an out-lens style and asnorkel-type (semi in-lens style). Moreover, a retarding style can beadopted, which applies a negative voltage to a sample to reduce thespeed of the primary electron beam. Still furthermore, each lens may beconstituted by an electrostatic lens constituted by a plurality ofelectrodes.

The primary electron beam 4 is scanned on the sample 10 by a scanningcoil 9 two-dimensionally. A secondary signal 12 such as secondaryelectrons generated from the sample 10 by radiation of the primaryelectron beam advances to an upper portion of the objective lens 7 andthereafter is separated from the primary electrons by an orthogonalelectromagnetic field (EXB) generator 11 for a secondary signalseparation. The secondary signal is detected by a secondary signaldetector 13. The signal detected by the secondary signal detector 13 isamplified by a signal amplifier 14 and then transferred to an imagememory 25 to be displayed as a sample image on an image display device26.

A one-stage deflection coil 51 (an aligner for the objective lens) isdisposed in the vicinity of the scanning coil 9 or at the same positionthereof and operates as the aligner for the objective lens. An octupoleastigmatism correction coil 52 (astigmatism corrector) for correctionastigmatism in the X and Y-directions is disposed between the objectivelens and the diaphragm plate. An aligner 53 for correcting an axisdeviation of the astigmatism correction coil is disposed in the vicinityof the astigmatism coil or at the same position thereof.

It is possible to allow the image display device 26 to display a buttonfor instructing a confirmation of axis conditions and a start of anautomatic axis alignment in addition to the sample image and variousoperation buttons for setting an electron optical system and scanningconditions.

When a focus adjustment is performed in a state where the primaryelectron beam passes through a position deviating from a center of theobjective lens, that is, in a state where an axis of the objective lensdeviates, a motion of a field of view occurs accompanied with a focusadjustment. When an operator notices the axis deviation, he/she caninstruct a start of an axis alignment processing by an operation such asclicking a processing start button displayed on the display device witha mouse. Upon receipt of the instruction of the axis alignment from theoperator, the computer 40 starts the procedures according to theflowchart described in the following embodiment.

Although the descriptions of FIG. 1 were made on condition that thecontrol processing section is provided integrally with the scanningelectron microscope or provided so as to form the similar to thisconstitution, the arrangement of the control processing section is, as amatter of course, not limited to this, and the processing describedbelow may be performed by use of a control processor provided separatelyfrom the scanning electron microscope barrel. When such a constitutionis adopted, a transmission medium which transmits a detection signaldetected by the secondary signal detector 13 to the control processorand transmits a signal from the control processor to a lens and adeflector of the scanning electron microscope is necessary as well as aninput/output terminals which input and output the signal transmitted viathe transmission medium. Moreover, a program for executing theprocessing described below may be previously registered in a recordingmedium, and this program may be executed by a control processor whichhas an image memory and supplies a signal necessary for the scanningelectron microscope.

The sample 10 is set on a stage 15. The stage 15 is moved by a controlsignal from the computer 40, whereby the primary electron beam 4 canmove to any position on the sample or the stage. An exclusive pattern 16for performing a beam alignment can be disposed on the stage.

It is possible to previously set automatic operation conditions by theimage display device 26 and an input device 42 such as a mouse and akeyboard. The automatic operation conditions are stored in a storagedevice 41 as a recipe file. In the recipe file, conditions for executingthe automatic axis alignment are also included.

Embodiment 1

The processing flowchart of FIG. 2 will be described in detail below.

First Step:

The present conditions of the objective lens 7 or conditions determinedbased on the present conditions, for example, conditions in which afocus is slightly deviated from the present focus conditions, are setfor the objective lens 7 as the condition 1. Next, the presentconditions of the aligner 51 or conditions determined in advance are setas the conditions 1 of the aligner 51. An image 1 is acquired by theobjective lens condition 1 and the aligner condition 1.

Second Step:

The condition of the aligner 1 is kept intact, and only the objectivelens condition is set to second focus condition in which a focusdeviates from the objective lens condition 1 by a value previouslydetermined, thus obtaining an image 2.

Third Step and Fourth Step:

A condition in which the condition of the aligner 51 is changed by apredetermined value for the condition 1 is set in the aligner 51 as thecondition 2. Then, the condition of the objective lens is set to theconditions 1 and 2 similarly to the steps 1 and 2, thus respectivelyobtaining images (image 3 and image 4).

Fifth Step:

An image is acquired again under the same condition as the image 1 andregistered as an image 5.

Sixth Step:

Parallax (image deviation) between the images 1 and 2 is detected by animage processing and registered as parallax 1. For example, an imagecorrelation is obtained while shifting the pictures of the images 1 and2 from each other at a pixel unit, and the parallax between the imagescan be detected based on a shift amount of one picture from the other atwhich an image correlation value becomes maximum. In addition, if animage processing capable of detecting parallax is adopted, this imageprocessing can be applied to this embodiment.

Seventh Step:

Parallax between the images 3 and 4 is detected by the image processingand registered as parallax 2.

Eighth Step:

Parallax between the images 1 and 5 is detected by the image processingand registered as parallax 3. Because the images 1 and 5 are acquiredunder the same condition, if there is a difference (parallax 3) betweenthese images, this difference is produced by a drift of the sample and abeam. Specifically, when the optical condition of the charged particlebeam is rendered to a certain state (first state), and then the opticalcondition is rendered to another state (second state), followed byrestoring the optical condition to the first state, the sample imagesare respectively detected under the above described two first states,and the drift is calculated based on the difference between both of thesample images.

Ninth Step:

A drift component is detected from the parallax 3, and the driftcomponent is corrected (removed) for the parallax 1 and the parallax 2.For example, if a fetching interval of the images 1 and 5 is t seconds,a drift (d) per unit time is represented by d=(parallax 3)/t. On theother hand, when the fetching interval of the images 1 and 2 is set toT12 and the fetching interval of the images 3 and 4 is set to T34, thedrift components of d×T12 and d×T34 are included in the parallaxes 1 and2, respectively. Therefore, by extracting the drift component from theparallaxes 1 and 2, a precision parallax resulting from the axisdeviation can be calculated.

Tenth Step and Eleventh Step:

An optimum value of the aligner 51 is calculated based on the parallaxes1 and 2 which have been subjected to the drift correction and set to thealigner.

Although the processing flowchart of FIG. 2 was described according tothe procedures that can be understood easily, the fetching order of theimages do not influence the processing except for the first and lastimages for correcting the drift. In an actual processing, to speed upthe processing, the objective lens 7 condition is set to the condition1, and the images 1 and 3 can be continuously fetched. Next, theobjective lens 7 condition is set to the condition 2, and the images 2and 4 can be continuously fetched. Because an objective lens of anelectron microscope is usually constituted by a magnetic lens and haslarge inductance, a method of continuously controlling an aligner whichhas less inductance and can perform a high speed control is practicallyeffective.

A principle by which an axis deviation relative to the objective lens iscorrected (amended) according to the processing flowchart of FIG. 2 willbe described with reference to FIG. 3. Assuming that, in a state of theaxis deviation, a beam deviation axis amount at a position (deflectedplane) of the aligner 51 be WAL (complex variable: XAL+j·YAL, j:imaginary unit) and a slant of the beam relative to the optical axis atthis position be WAL′(complex variable), an orbital calculation based onan electro-optics theory (paraxial theory) is available. In the case ofa magnetic objective lens, assuming that an image deviation amount(parallax) produced when a lens current value is changed from I1 to I2by ΔI(=I1−I2) be ΔWi (complex variable: ΔXi+j·ΔY), ΔWi can be expressedby the following formula,

ΔWi=K·ΔI·(WAL·A+WAL′·B)  (1)

where K, A and B are parameters (complex numbers) determined by an axisdeviation state in measurements and operation conditions of theobjective lens (such as an acceleration voltage, a focal distance or aposition of the objective lens). The state where the axis deviatesrelative to the objective lens means that ΔWi has a value other thanzero in the formula (1). Accordingly, conventionally, the current of theobjective lens was changed periodically by ΔI, and the operatorrecognized the image deviation ΔWi. He/she adjusted the condition of thealigner to removed the image deviation. Specifically, an optimum valueof the aligner for which the axis deviation is corrected indicates acondition in which the right side of the formula (1) is zeroirrespective of ΔI. When this condition is written in the form of aformula, the following formula (2) is obtained.

(WAL·A+WAL′·B)=0  (2)

An operation condition of the aligner satisfying this condition is theoptimum value. If the axis deviation exists, because a tilt of anincidence beam exists in the aligner deflected plane, this tilt isexpressed as WAL0′, and a deflection angle (controlled value) by thealigner is expressed as WAL1′. The tilt of the beam relative to theoptical axis is expressed by the following formula (3).

WAL′=WAL0′+WAL1′  (3)

Accordingly, to obtain the condition WAL1′ (the optimum value of thealigner) of the aligner satisfying the formula (2) is an object of theaxis alignment function. In the case where the aligner is constituted byan electromagnetic coil, the deflection angle WAL1′ is in proportion toa coil current of the aligner. When the formula (1) is rewritten usingthe above described relations, the following formula (4) is obtained.

ΔWi=ΔI·(A1+WAL1′·B1)  (4)

In the formula (4), A1 and B1 are expressed by the following formulas(5) and (6).

A1=K·(WAL·A+WAL0′·B)  (5)

B1=K·B  (6)

From the formula (4), the optimum value WAL1′ of the aligner is given bythe following formula (7).

WAL1′=−A1/B1  (7)

Accordingly, it is possible to calculate the optimum value of thealigner by obtaining A1 and B1. In the formula (4), since ΔI is thecurrent change amount of the objective lens, ΔI can be determinedpreviously as a known value. Accordingly, the aligner is set toarbitrary two conditions previously determined and, for each of theseconditions, the parallax ΔWi relative to ΔI is detected by the imageprocessing. At this time, formulas for obtaining the unknowns A1 and B1from the formula (4) can be obtained. Since the unknowns A1 and B1 canbe solved based on these formulas, the optimum condition of the alignercan be determined based on the formula (7).

Specifically, a formula of n-degree having the unknowns such as A and Bis solved under the condition that the parallax ΔWi, which is obtainedwhen the aligner is set to the arbitrary two conditions previouslydetermined, becomes small (ideally zero), whereby a condition can bededuced which does not depend on the operation condition of the electronoptical system. The aligner condition, that is, an excitation conditionof the aligner, can be deduced based on this condition. Note that thealigner 51 has an arrangement or a structure which is capable ofcontrolling a beam passage position two-dimensionally at least in a mainplane of the objective lens. This is because if a deflection fulcrum ofthe beam by the aligner exists in the vicinity of the main plane of theobjective lens, a state of the axis deviation relative to the objectivelens cannot be controlled. Specifically, in the case of the alignmentdeflector (aligner) using the electromagnetic coil like this embodimentof the present invention, it is possible to detect an excitation current(deflection signal) supplied to the coil, which changes depending on anoptical condition. For example, since the excitation current, whichchanges depending on a change of an excitation condition of theobjective lens and depending on a level of a retarding voltage appliedto the sample, can be detected based on the optical conditions inobservation, it is unnecessary to register parameters different for eachoptical condition previously. Even if a condition of the beam changes bya change due to the passage of time, an excitation current supplied to aproper alignment coil in the state where the beam condition changes canbe detected.

As described above, this embodiment of the present invention can copewith a changing state of the axis deviation and operation conditions ofan optical device in the charged particle optical system, which includebeam energy, a focus distance and an optics magnification, and it ispossible to realize an automatic axis alignment easily.

The magnitude of the axis deviation can be quantized by the magnitude ofthe parallax ΔWi relative to ΔI. Therefore, when an operation such as asample exchange and a condition change of the electron optical system,which may cause the axis deviation, is performed, the axis deviation canbe detected previously as long as a processing to detect the parallaxΔWi due to ΔI is executed. Moreover, when the parallax ΔWi exceeds apredetermined value, a message can be displayed, which informs theoperator that he/she must perform the axis alignment. An example of amessage screen when the axis deviation is detected is shown in FIG. 5.The operator can execute the axis alignment processing by the inputmeans according to this message if necessary. The input means can adoptvarious styles in which, for example, icons displayed on the messagescreen (for example, FIG. 5) and other exclusive icons displayed on amonitor are clicked by use of a mouse or a processing command isdesignated from a menu screen.

Embodiment 2

On the other hand, with respect to the astigmatism corrector 52, anautomatic axis alignment is possible in this embodiment. In theastigmatism corrector, on a plane perpendicular to the optical axis, anaction to converge the beam and an action to diverge the beam aregenerated in different directions. Accordingly, if the beam does notpass through the center of an astigmatism correction field, the beamwill be subjected to a deflection action in a direction corresponding tothe deviation from the center of the astigmatism correction field. Atthis time, because the deflection action also changes in associationwith the correction of the astigmatism, the image moves in associationwith the alignment operation for the astigmatism, and the alignmentoperation is difficult. To correct the movement of the image,conventionally, a signal in association with a signal (Xstg, Ystg) ofthe astigmatism corrector 52 was input to another aligner 53, and themovement of the image by the astigmatism corrector was cancelled by amovement of the image generated by the aligner 53. At this time, whenthe signal (complex variable) input to the aligner 53 is expressed byWs1, Ws1 is expressed by the following formula (8),

Ws1=Ksx·Xstg+Ksy·Ystg  (8)

where Ksx and Ksy are coefficients represented by complex variables.Now, assuming that the signal (Xstg, Ystg) of the astigmatism correctoris changed individually by ΔXstg and ΔYstg, motions (parallax) ΔWix andΔWiy of the observed image corresponding to each change are respectivelyrepresented by the following formulas (9) and (10),

ΔWix=ΔXstg·(Asx+Bx·Ksx)  (9)

ΔWiy=ΔYstg·(Asy+By·Ksy)  (10)

where Asx and Asy are complex variables having values determined so asto correspond the axis deviation of the beam corresponding to theastigmatism correction. Ksx and Ksy represent axis alignment parameters(complex variables) controlled by the apparatus. Moreover, Bx and By arecomplex variables determined by the position and deflection sensitivityof the aligner, the condition of the electron optical system and thelike. Conventionally, modulation signals of ΔXstg and ΔYstg wererespectively added to the astigmatism corrector, and the operatorrecognized the movements (ΔWix, ΔWiy) at this time. Thus, the operatorperformed a manual adjustment for the parameters Ksx and Ksy to cancelthe movements (ΔWix, ΔWiy).

This is the axis alignment operation for the astigmatism corrector.Specifically, the operation to align the axis to the astigmatismcorrector corresponds to finding the coefficients Ksx and Ksy at whichΔWix and ΔWiy becomes zero irrespective of ΔXstg and ΔYstg in theformulas (9) and (10). Although ΔWix and ΔWiy should be zero ideally, away to find the coefficients is not limited to the above, and thecoefficients may be found under conditions that ΔW is made small so asto be close to zero. The form of the formulas (9) and (10) is entirelythe same as that of the formula (4) shown in the above, and when thechange of the current value (ΔI) of the objective lens is substitutedfor a signal change (Δ Xstg, Δ Ystg) of the astigmatism corrector, it ispossible to find optimize control parameters (Ksx, Ksy) for the aligner53 by the parallax detection and the calculation processing. Theprocessing flowchart for finding the optimum control parameters is shownin FIG. 4. Since the aligner for correcting the deviation of the fieldof view by the astigmatism corrector serves for correcting the positionof the beam on the sample, the aligner must be disposed at a positionwhere the position of the beam on the sample can be controlled.

The magnitude of the axis deviation relative to the astigmatismcorrector can be quantized by the image deviation (parallax) when thechanges ΔXstg and ΔYstg are added to the signal of the astigmatismcorrector. For this reason, in this embodiment, when an operation suchas a change of an acceleration voltage, a sample exchange and a changeof a focus position, which may change a state of the optical axis, isperformed similarly to the foregoing case the axis deviation relative tothe objective lens, the parallax detection is performed, and it ispossible to inform the operator of the state of the axis deviation bydisplaying it. The operator follows this displaying and can instruct theastigmatism corrector to execute the axis alignment processing by theinput means displayed on the screen if necessary. The input means canadopt various styles in which, for example, exclusive icons displayed ona monitor are clicked by a mouse, or a processing is designated from amenu screen.

In this embodiment of the present invention, it is possible to preventan erroneous processing operation when the operator instructs the axisalignment processing erroneously in a state where the image is improper,that is, a state where the focus remarkably deviates, and a state of animage which includes almost no structural information. Descriptions forthis function will be made by use of the processing flowchart of FIG. 6.When a processing for detecting the axis deviation or a start of aprocessing for the axis alignment is instructed, the CPU 40 fetches thepresent image thereinto and executes a quantization (image qualityquantization) processing for the fetched image. The processing by thequantization means is executed for quantizing as to whether an imageincludes structural information necessary for a parallax detection. Asan output of this processing, for example, the image is subjected to aFourier transform, and, based on this result, a quantization value Ficalculated by the following formula (11) can be used.

Fi=ΣΣ[F(fx,fy)·fx ^(n) ·fy ^(n)]  (11)

where F(fx, fy) represents a two-dimensional Fourier transform (FFT) ofthe image, and fx and fy represent a spatial frequency. By using a realnumber and an integer equal to one or more as an index number n, aproper quantization for the image quality is possible. Specifically, ifthe image includes no structural information, F(fx, fy) is very smallvalue in a region where fx and fy are larger than zero. Accordingly,based on the calculation result of the formula (11), it is possible todecide whether the image quality includes proper structural information.In the case where this quantization value Fi is equal to a valuepreviously determined or less or lower than this value, an alarm shouldbe issued based on a decision that the quantization value Fi is notproper for an alignment signal calculation. This alarm may be a displayas shown in FIG. 5 or sound.

Embodiment 3

FIG. 7 is a drawing for explaining a third embodiment of the presentinvention, which shows a setting screen for setting environments for anautomatic axis deviation correction displayed in an image displaydevice. An operator of a scanning electron microscope sets theenvironments of the automatic axis alignment based on this screen. Inthe case of this embodiment, an example in which the environments areset by use of a pointing device 60 on the setting screen will bedescribed. First, the operator decides whether an aperture alignment isexecuted automatically, and selects any one of “Correction Based OnParallax Detection”, “Previously Determined Value Correction” and “NoCorrection”. “Correction Based On Parallax Detection” is a mode in whichthe axis deviation correction is executed in the steps described inEmbodiment 1. If this mode is selected, axis correction precision, whichis stable for a long time regardless of a change of a primary electronbeam due to the passage of time, can be acquired. “Previously DeterminedValue Correction” is a mode in which an exciting condition of anobjective lens and an axis deviation caused for each of distancesbetween a sample and the objective lens, which are a plurality ofoptical conditions such as a working distance, are previously registeredin a memory (not shown), and an axis alignment is performed under theregistered axis alignment condition when a predetermined opticalcondition is set. This mode should be selected, for example, when nochange of the axis deviation depending on the passage of time occurs andwhen approximately the same axis deviation is recognized regardless ofthe change of the optical condition. Since in this setting thecorrection is performed based on the previously determined value, adetection of the axis alignment condition and a calculation time are notneeded, and hence shortening of the processing time is possible. “NoCorrection” is a mode in which the axis alignment is not performed, andthis mode should be selected under an environment where the axisdeviation does not occur.

As described above, if a preparation is made so as to be capable ofselecting a plurality of correction modes by an environment settingscreen, it is possible to select a proper correction condition based onthe usage condition and the environment of the scanning electronmicroscope.

Next, the operator selects an automatic axis alignment timing. Withregard to this selection, for example, when a frequency of occurrence ofthe axis deviation is high, “Every Analysis Point” is set inconsideration for precision of the axis alignment, and the axisdeviation correction is performed for each measurement point. When theaxis deviation does not occur so frequently, “Every Wafer” is selectedin consideration for throughput, and the axis deviation correctionshould be performed every time when a wafer to be measured by thescanning electron microscope is replaced with another. By providing sucha chance of selections, it is possible to select a proper timing of theaxis deviation correction based on the usage condition and theenvironment of the scanning electron microscope. Furthermore, when “WhenParallax Exceed Predetermined Value” is selected, the parallax ΔWi forthe objective lens current change amount ΔI is detected for eachanalysis point or each wafer. When the parallax ΔWi exceeds thepredetermined value, “Correction Based On Parallax Detection” isperformed. In addition, when “User Setting” is selected, the axisalignment is performed at an axis alignment timing separately registeredin advance.

Next, the operator selects whether he/she should register a correctionamount graph or not. The correction amount graph is displayed in theform as shown in FIG. 8( a) on the image display device. In thetechnology described in Embodiment 1, a coil current supplied to theastigmatism correction aligner 53 is calculated finally. A differencebetween the magnitude of the coil current and the magnitude of thenon-corrected coil current represents how far the beam deviates from theoptical axis, and it is possible to judge a transition of a degree ofthe axis deviation with a graph of the degree of the axis deviation madeby plotting it. If this transition of the axis deviation showsapproximately a certain value, the mode is switched to “PreviouslyDetermined Value Correction” formerly selected based on the judgmentthat a later state of the axis deviation is unchanged, whereby adetection time and a calculation time of the axis alignment condition,which are required for “Correction Based On Parallax Detection”, can bedeleted and thus throughput can be enhanced. Displaying of such a graphcan allow the operator to make a judgment for a proper automatic axisalignment and to set a proper axis alignment condition.

The graph shown in FIG. 8( b) is an example in which measurement resultsfor semiconductor pattern widths are displayed so as to be superimposedon the correction amount graph of FIG. 8( a). The measurement of thesemiconductor pattern widths is performed by measuring the width of aline profile formed based on a detection amount of secondary electronsand reflected electrons, which are obtained by scanning an electron beamone-dimensionally or two-dimensionally on a semiconductor device onwhich a pattern to be measure is formed. The measurement results of thepattern to be measured obtained in the above described manner and errorsof pattern dimensions based on design information are plotted so as tobe superimposed on the correction amount graph shown in FIG. 8( a).

In FIG. 8( b), the symbol “A” indicates a point where the measurementwas performed under the condition that “Correction Based On ParallaxDetection” is not performed because the parallax ΔWi exceeds a certaindetermined range or because there is no structural information necessaryfor the parallax detection (in the case where the quantization value Fidescribed in Embodiment 2 is equal to a value or less or lower than thisvalue). In order to make it possible to distinguish this portion from aportion where the correction amount is zero, this portion should bedisplayed in such a manner that this portion is distinguished from otherportions by displaying this portion with a different color. In thefollowing descriptions, when the parallax ΔWi exceeds a predeterminedrange, descriptions for the case where the measurement is executedwithout performing “Correction Based On Parallax Detection” will bemade. However, the way of the measurement is not limited to this, and analarm to urge the operator to perform the axis alignment or the like maybe issued so that the operator stops the automatic measurement. Notethat when the measurement is continued in spite that “Correction BasedOn Parallax Detection” is not performed, the obtained measurement valuemay be erroneous. In such a case, in order to confirm with eyes laterwhether the measurement was properly performed, at least one of thesample image obtained in the measurement, the line profile and theoptical condition of the electron microscope should be memorizedtogether with the measurement value. The operator can judge reliabilityof the measurement by checking these information against the obtainedmeasurement results.

Next, when the parallax ΔWi exceeds a certain determined range or whenthe setting value Fi is equal to a certain value or less, or less thanthis certain value, the operator selects which processing he/she shouldperform. When the operator selects “Measurement Stop”, the measurementthat is being executed automatically and continuously falls in a stopstate, and the electron beam is blanked by a blanking mechanism (notshown) so as not to be radiated onto the sample. Thus, a standby stateis brought about. At this time, the message shown in FIG. 7 may bedisplayed on the image display screen. Among the modes, “Continue”represents a mode in which “Correction Based On Parallax Detection” isnot performed but the measurement is performed continuously. “ContinueAfter Sample Image Registration” is a mode in which the sample image andthe like obtained without performance of “Correction Based On ParallaxDetection” described above is registered together with the measurementresults. “Switching To Correction Of Previously Determined Value” iseffective such as when “Correction Based On Parallax Detection” cannotbe performed and a state of the axis deviation is found out to someextent. In this mode, the axis alignment is performed based on thepreviously registered correction amount. Moreover, without performanceof the measurement, skipping to a next measurement point may be done. Asa matter of course, the environment setting screen described so far canbe applied to the astigmatism alignment.

In order to decide whether the automatic axis alignment described inthis embodiment is performed properly, at least four sample imagesserved to perform “Correction Based On Parallax Detection” may bedisplayed on the image display screen in real time. Although the abovedescriptions were made for explaining the performance of the axisalignments for the objective lens and the astigmatism corrector, theaxis alignment is not limited to the above, but the axis alignment isgenerally applicable to optical devices of the charged particle beam,for which the optical axis alignment must be performed by use of thealignment deflector. Moreover, the present invention can be applied toall of charged particle beam apparatuses which converge a chargedparticle beam by use of a convergence ion beam and an axis symmetry lenssystem. In addition, an electrostatic deflector may be employed as thealigner deflector.

Embodiment 4

Next, in an apparatus especially desired to be operated automatically,to which many samples are continuously introduced, which includes ascanning electron microscope for measuring dimensions of a pattern widthformed on a semiconductor wafer and a contact hole and a scanningelectron microscope which checks existence of defects on thesemiconductor wafer and reviews the detected defects, a preferredembodiment for performing an axis alignment for an optical device suchas an objective lens and an astigmatism corrector for aligning anelectron beam, which are incorporated in each of the above scanningelectron microscopes, will be made.

FIGS. 9 and 10 are flowcharts for explaining the embodiment, and theflowcharts are executed according to a program previously stored in astorage device 41 and a command input from then input device 42. Theflowchart shown in FIG. 9 differs from the flowcharts shown in FIGS. 2and 4 in that while the technique of the axis alignment is unchanged inthe flowcharts shown in FIGS. 2 and 4, the technique of the axisalignment changes in accordance with the states of affairs in theflowchart shown in FIG. 9.

In Step 2001, an initial value A0 (for example, the present condition 1)of the adjustment aligner such as the objective lens aligner 51 and theastigmatism correction aligner 53 is acquired and stored in the computer40. In Step 2002, an image movement W1 which means parallax in theembodiments 1 to 3 is calculated. The calculation of an amount of theimage movement is executed in Steps 3001 to 3006 to be described later.In Step 2003, it is decided whether η is recalculated by a flagpreviously given. η herein is an unknown to be found in this embodimentas described later. When the recalculation is performed, Steps 2004 to2006 are executed. When the recalculation is not performed, the imagemovement W2 is set to zero, and a previously given value is set to η.Thereafter, Step 2011 is executed. In Step 2004, a condition (condition2), in which a deviation amount ΔA1 is changed relative to the initialvalue A0 of the aligner stored in the computer 40, is set in thealigner. In Step 2005, the image movement W2 is calculated according tothe processing flowchart of Steps 3001 to 3006.

Next, in Step 2006, η is calculated from the formula (12) by use of theimage movement W1 and the image movement W2 stored in the computer 40.

η=−1/(W2−W1)  (12)

In Step 2007, it is decided whether ε is recalculated by a flagpreviously given. Herein, ε is a constant inherent to the apparatus,which is to be found in this embodiment as described later. When therecalculation is performed, Steps 2008 to 2010 are executed. When therecalculation is not performed, the image movement W3 is set to zero,and a previously given value is set for ε. Thereafter, Step 2011 isexecuted. In Step 2008, a condition (condition 3 different fromconditions 1 and 2 described above) in which a deviation amount ΔA2 ischanged relative to the initial value A0 of the aligner stored in thecomputer 40 is set in the aligner In Step 2009, the image movement W3 iscalculated according to the processing flowchart of Steps 3001 to 3006.

In Step 2010, E is calculated from the formula (13) by use of the imagemovements W1, W2 and W3 stored in the computer 40.

ε=(W3−W2)/(W2−W1)  (13)

In Step 2011, alignment correction values X and Y are calculated by useof the image movement W1, η, ε and |ΔA1 | according to the formula (14),and the alignment correction values X and Y are set in the aligner.

X+jε·Y=|ΔA1|η·W1  (14)

In Step 2012, when an absolute value (X·X+Y·Y) of the alignmentcorrection value, that is, an actual axis deviation amount, is equal toa threshold previously determined or larger than the threshold, aretrial processing (Steps 2001 to 2012) is performed. The retrialprocessing compensates deviation correction precision when an image istaken in and the deviation detection is performed in a state where theinitial alignment significantly deviates. By repeating the correctionsplural times in the above described manner, the deviation can becorrected with higher precision.

Next, a calculation step for the image movement will be described by useof FIG. 10. In Step 3001, an initial value (for example, the presentcondition) C0 of the coil to be adjusted (the objective lens 7 or theastigmatism corrector 52) is acquired and stored in the computer 40. InStep 3002, a condition, in which as the condition 1 a value ΔCpreviously determined is changed relative to the initial value C0 of thecoil to be adjusted, is set for this coil. In Step 3003, the image 1 isacquired in the condition 1 and stored in the image memory 25. In Step3004, a condition, in which as the condition 2 a value −ΔC previouslydetermined is changed relative to the initial value C0 of the coil to beadjusted, is set for this coil.

In Step 3006, the image movement W is calculated in an image processingdevice 27 based on the images 1 and 2 and stored in the computer 40. Theimage movement W is a vector of (x, y) and a deviation amount of theimages 1 and 2. With respect to the calculation for the deviationamount, a position which is most similar to the image 2 using a partialimage of the image 1 as a template is calculated by use of the followingformula (15).

$\begin{matrix}{{r\left( {X,Y} \right)} = \frac{\begin{bmatrix}{{N{\sum\limits_{i,j}{Pi}}},{jMi},{j -}} \\{\left( {{\sum\limits_{i,j}{Pi}},j} \right)\left( {{\sum\limits_{i,j}{Mi}},j} \right)}\end{bmatrix}}{\sqrt{\begin{matrix}\left\lbrack {{N{\sum\limits_{i,j}{P^{2}i}}},{j - \left( {{\sum\limits_{i,j}{Pi}},j} \right)^{2}}} \right\rbrack \\\left\lbrack {{N{\sum\limits_{i,j}{M^{2}i}}},{j - \left( {{\sum\limits_{i,j}{Mi}},j} \right)^{2}}} \right\rbrack\end{matrix}}}} & (15)\end{matrix}$

r(X, Y) is a correlation value in (X, Y), and Pij is a density value ata point (X+_(i), Y+_(j)) of the image 1 corresponding to the image 2.Mij is a density value in a point (X+_(i+1), Y+_(j+1)), and N is thenumber of pixels of a pattern mask. The deviation value to be found isequal to a value obtained by subtracting (X, Y) from the position of thepartial image of the image 1. This method shows a high degree of freedombecause any pattern may be used in this method.

As another method for calculating the image movement, the following isconceived. When an image for calculating the image movement includes aspecific shape, for example, when a holder pattern is included in theimage, the position of the pattern in each of the images 1 and 2 isdetected according to the following method, and the deviation amount canbe detected.

First, the image 1 is subjected to a differentiation by adifferentiation filter, and a threshold is set so that an edge is left.Thus, a binary screen is prepared. This binary screen undergoes asegment processing, and only an edge forming a pattern is extracted. Thecenter of gravity (x1, y1) of the pattern is calculated based on theedge information extracted. The same processing is performed for theimage 2, and the center of gravity (x2, y2) of the pattern iscalculated. The deviation amount to be found becomes equal to W(x2−x1,y2−y1). Even if a pattern having a circular shape originally is detectedto be elliptical by changing an electron optical condition, thistechnique shows a merit that this technique is tough against a change inshape of the pattern because the position of the center of gravity ofthe pattern hardly change.

The processing flowchart described herein is the same as those for theobjective lens aligner 51 and the astigmatism correction aligner 53(X-direction, Y-direction) except for control values. Moreover, ε is aconstant inherent to an apparatus disposed in the X and Y-directions,which concerns a sensitivity difference and an orthogonal deviation.Accordingly, values which were found periodically and at the time ofstarting-up of the apparatus are previously stored in the storage device41. The stored values are previously read into the computer 40 beforethe processing flowchart is executed, whereby Steps 2008 to 2010 can beomitted. When the axis alignment is performed in a relatively shortcycle without a change of the electron optical condition, the valuecalculated for η last time is previously stored in the computer 40, andthe calculated value can be used as η.

As described above, this embodiment has a feature in that six pieces ofimages obtained by changing the optical condition are used, and the modein which the predetermined variables such as ε and η are recalculated(hereinafter referred to as a three-point measurement mode), the mode inwhich only η is recalculated based on four pieces of images obtained bychanging the optical condition hereinafter referred to as a two-pointmeasurement mode) and the mode in which and η are not recalculated(hereinafter referred to as a one-point measurement mode) arealternatively used according to the sate of affairs. While thethree-point measurement mode is capable of obtaining high axis alignmentprecision, two pieces of images will do in the one-point measurementmode, and a high processing speed can be achieved. Since each of modesshows an advantage inherent to them respectively, for example, it isdesirable to selectively use each mode as described below.

The three-point measurement mode is performed, for example, at the timeof starting up a semiconductor testing apparatus, and ε is previouslycalculated at that time. The two-point measurement mode is executed oncea day or at the time of a change of a recipe for changing a condition ofthe semiconductor testing apparatus significantly, and ε is used afterreading out it from the storage device 41. The one-point measurementmode is executed for each measurement point of the semiconductor waferto be checked, and ε and η are executed after reading out them from thestorage device 41 and the computer 40, respectively. Needless to say,the above descriptions were made for a mere example, and, as a matter ofcourse, various modifications can be employed according to sorts of theapparatus and measurement conditions.

Note that since the deviation amount ΔA uses the sample image fordetecting the image movement, the following two conditions must besatisfied. (1) The deviation amount must be made much to a certainextent so that the movement of the sample pattern can be recognized. (2)The deviation amount must be made less to a certain extent so that thesample pattern is previously out of the screen. The conditions (1) and(2) can be determined if a geometrical position of the sample pattern isfound. Specifically, the deviation amount ΔA is determined based on thegeometrical arrangement of the sample pattern and an observationmagnification. This deviation amount ΔA may be determined by providingan automatic sequence so as to be less when the magnification is highand much when the magnification is low. Alternatively, the deviationamount ΔA may be input automatically from the input device 42.

According to this embodiment of the present invention, in the apparatuswhich performs the axis alignment for the charged particle opticalsystem based on the obtained sample image, the calculation means asdescribed above and the selection means for selecting one of theplurality of axis alignment methods (the plurality of calculationmethods) are provided, whereby it is possible to realize compatibilitybetween the high axis alignment precision and the high processing speed.Such technical effects are particularly effective for the semiconductortesting apparatus to which the semiconductor wafer having the pluralityof measurement points is continuously introduced, in which there is apossibility of the change of the optical condition with the passage oftime because of the continuous automatic operation, and in which theoptical condition changes by changing the recipe. In this technology, aproper axis alignment method can be assigned according to demand.

The parameter η adopted in this embodiment represents in what way theimage movement amount (including the direction) changes when thealignment coil is operated and is one including the alignment deflectionsensitivity. Note that the parameter η changes depending on theoperation condition of the electron optical system as well as dependingon the simple alignment deflection sensitivity.

In this embodiment, the basic formula (1) described in the foregoingembodiment is transformed as described below, and the parameter in theformula (1) is replaced with the parameter η. The tilt of the electronbeam orbit in the alignment coil described in the former embodimentincludes two tilts: one results from the axis deviation (WAL0′) and theother results from the deflection (WAL1′) owing to the present settingvalue of the alignment coil. Moreover, assuming that a tilt of a beamrelative to a changed amount (setting changing value) of the presentsetting value of the alignment coil be WAL2′, the following formula (16)is given.

WAL′=WA0′+WAL1′+WAL2′  (16)

Because a parameter necessary for the control is WAL2′ in the formula(16), the formula (1) is represented by the following formula (17) whenthe terms other than WAL2′ are assumed to be a constant.

ΔW=ΔI·K·(A1+B1·WAL2′)  (17)

Here, an image movement amount given by ΔI·K·A1 corresponds to the imagemovement amount caused under the condition of the present setting valueof the alignment.

On the other hand, a relation between a DAC value (X, Y) of thealignment coil and the tilt WAL2′ of the beam can be written by thefollowing formula (18),

WAL2′=k·(X+jε·Y)  (18)

where k is a coefficient representing the sensitivity of the alignmentcoil, and ε represents a complex relative sensitivity of Y relative to X(absolute value of ε: sensitivity ratio, arg(ε): orthogonal deviation.The formula (18) is substituted for the formula (17), and meaninglesscoefficients are collected to be represented. The image movement amountΔW when the objective lens current is changed can be written by thefollowing formula (19).

ΔW=A2+B2·(X+jεY)  (19)

Since a condition of a central axis of current is that ΔW is equal tozero, an alignment value satisfying this condition is calculated by thefollowing formula (20).

X+jεY=−A2/B2  (20)

Accordingly, if A2 and B2 are deduced from the image movement amount,the alignment control value (X, Y) by which the center axis of currentis obtained can be calculated. To calculate A2 and B2, the imagemovement amount W1 when X=Y=0 in the formula (17) and the image movementamount W2 when X=X1≠0 and Y=0 in the formula (17) are detected.Specifically, the following formulas (21) and (22) are obtained.

W1=A2  (21)

W2=A2+B2X1  (22)

From the formulas (21) and (22), the formula (20) is written by thefollowing formula (23).

X+jεY=−X1·W1/(W2−W1)  (23)

In the embodiment, the term of −1/(W2−W1) in the formula (23) is definedas η. When η is rewritten, the following formula (24) is obtained.

η=−1/(W2−W1)=−1/B2  (24)

Embodiment 5

FIG. 11 is a flowchart for explaining an embodiment concerning afull-automatic axis alignment. The full-automatic axis alignment in thisembodiment is to perform automatic controls including a series of thefollowing operations. Specifically, a stage 15 is driven at timingspreviously determined, and a pattern 16 for alignment is positioned justbelow an electron beam, followed by setting a magnification and an imagepickup based on pattern information. Thereafter, for example, anastigmatism correction aligner 53 is adjusted as to its X-direction, andthen the astigmatism correction aligner 53 is adjusted as to itsY-direction, followed by adjusting an objective lens aligner 51. Notethat, an adjustment order of the astigmatism correction aligner 53 andthe objective lens aligner 51 is determined depending on arrangements oflenses in an electron optical system. In the case of the electronoptical system illustrated in FIG. 1, when an axis alignment isperformed by the astigmatism correction aligner 53 after an alignment isperformed by the objective lens aligner 51, an optical axis of theelectron optical system sometimes deviates relative to the objectivelens again. Accordingly, it is desirable to begin the alignment at firstwith an optical device positioned closer to a negative electrode. In thecase of an electron optical system in which lenses are arranged in orderof the objective lens and the astigmatism corrector when viewed from thenegative electrode, the alignment should be performed in order of theobjective lens aligner and the astigmatism correction aligner.

In the descriptions of this embodiment, though an alignment pattern isprovided separately from the sample, this embodiment is not limited tothis, and the axis alignment may be performed by use of a specificpattern on the sample 10 such as a semiconductor wafer, which is anobject to be observed.

Details of the processing flowchart of the full-automatic axis alignmentwill be described by use of FIG. 11 and an automatic axis alignmentcondition setting screen 500 illustrated in FIG. 12. FIG. 12 isdisplayed in the image display device 26 as a screen for setting onecondition of the recipe file in which conditions of the automaticoperation are registered. The user sets the automatic axis alignmentcondition in this screen and begins the automatic operation.Descriptions for the processing flowchart of the full-automatic axisalignment during the execution of the automatic operation will bedescribed below.

In Step 4001 of FIG. 11, pattern information previously registered inthe storage device 41 is read out, and a deviation amount is calculatedbased on a magnification. Moreover, based on the measurement modedescribed in the former embodiment, E and 77 are initialized ifnecessary. With respect to the pattern information used for the axisalignment, an alignment pattern flag 502 or a wafer pattern flag 503 isselected, whereby it is decided which one of the alignment pattern 16 onthe stage and the pattern on the wafer is used. When the wafer patternflag 503 is selected, stage coordinates of the pattern, a sample imageacquisition magnification and the number of frames when the sample imageis acquired are input from numerical value input windows 504, 505 and506, respectively. When the alignment pattern flag 502 is selected, thestage coordinates, the magnification and the number of the frames, whichare previously stored in the storage device 41, are set to the numericalvalue input windows, respectively. Note that, the number of the framesset in Step 4001 means the number of times of totalization for thescanning images to form the image of the pattern. In this embodiment,one pattern is obtained by totalizing sixteen pieces of sample images.

In Step 4002, the stage coordinates 504 are fetched out from the patterninformation and then moved to the pattern position. When the alignmentpattern flag 502 on the stage is selected, the stage is moved so thatthe axis alignment pattern 16 is positioned just below the electronbeam. When the stage is moved, a value of current supplied from ascanning coil control power source 24 to the scanning coil 9 is set inaccordance with the magnification input from the numerical value inputwindow 505.

In Step 4003, a judgment for ON/OFF of an automatic focusing executionflag 501 is performed, and an automatic focusing is executed when theautomatic focusing execution flag 501 is decided to be ON. In Step 4004,images of the number of frames input from the numerical value inputwindow 506 are totalized, and the sample image is formed. In Step 4005,if an instruction to render an astigmatism correction aligner(X-direction) adjustment flag 507 ON is issued thereto, an adjustment ofthe astigmatism correction aligner (X-direction) is executed (Steps 2001to 2012, Steps 3001 to 3006). In Step 4006, if an instruction to renderan astigmatism correction aligner (Y-direction) adjustment flag 508 ONis issued thereto, an adjustment of the astigmatism correction aligner(Y-direction) is executed (Steps 2001 to 2012, Steps 3001 to 3006). Inthis adjustment, if a detection of the deviation amount is failed andthe automatic focusing execution flag 501 is OFF, the detection of thedeviation amount is tried once more after the focusing is executed.

In Step 4007, if an objective lens aligner adjustment flag 509 is ON, anadjustment of the objective lens aligner is executed (Steps 2001 to2012, Steps 3001 to 3006). In Step 4008, when a flag 511 a for thethree-point measurement mode is selected, ε is stored in the storagedevice 41, and η is stored in the computer 40. When a flag 511 b for thetwo-point measurement mode or a flag 511 c for one-point measurementmode is selected, η is stored in the computer 40.

In this embodiment, though each of the measurement modes is selected bythe flag previously determined, depending on a state of the imagemovement W1 calculated in Step 2002, for example, it may be decided bywhich mode the axis alignment is executed. For example, when the imagemovement W1 is large, the calculation based on many images is executed.The mode selection may be performed based on other information obtainednot only by the image movement but also by comparison of two images. Theselection of the measurement mode may be performed not only by aninstruction of the operator but also by an automatic operation.Specifically, selection means for selecting the calculation method ofthe present invention may be previously set by the operator as describedin the former embodiment. In addition, the selection means may functionto automatically change the calculation method for calculating thedeviation amount based on an evaluation of the image. In Step 4009, whena flag 510 for an automatic astigmatism alignment is ON, the automaticastigmatism alignment is performed.

When the automatic axis alignment is executed during the automaticoperation, the automatic axis alignment is executed while usuallyrendering all of the flags 501, 507, 508, 509 and 510 ON. When thesample moves a predetermined pattern position, a height of the sampledeviates in some cases from a focus position that has been adjustedbefore the movement. If the axis alignment is performed in the statewhere the height of the sample deviates from the position of the focus,the image deviation is detected with a pattern in a blurred image.Accordingly, axis alignment precision becomes lower. This problem can besolved by detecting the image deviation after the auto focusing isperformed like this embodiment.

Moreover, with respect to the three axis alignments for the astigmatismaligners (X and Y-directions) and the objective lens aligner, decisionas to which aligner makes a deviation is originally difficult as long asan operator is skilful. Therefore, even when the axis alignment isperformed manually, the axis alignments for all of the aligners areperformed in almost all of the cases. According to the embodiment of thepresent invention, since the control so as to automatically perform theoptical alignment in the most suitable order, that is, in order of{circle around (1)} the focus adjustment (auto focusing), {circle around(2)} the axis alignment for the astigmatism corrector (the axisalignment by the first alignment deflector), {circle around (3)} theaxis alignment for the objective lens (the axis alignment by the secondalignment deflector), and {circle around (4)} the astigmatismcorrection, the axis alignment can be executed with high precision andwith high throughput.

As shown in FIG. 12, if setting items of the recipe is previouslyarranged in the actual alignment order of the optical system, it ispossible to set the recipe while imaging an actual alignment performedin the electron optical system. Accordingly, there is a merit thatsetting is easy.

When the images used in each measurement mode described in the formerembodiments and this embodiment is displayed on the image display device26 in real time or after the image is once memorized in the image memory25, it is possible to confirm by a visual inspection whether the axisalignment is correctly performed. For example, when the axis alignmentis performed in a state where the focus is apparently defocused, ablurred image which is defocused is displayed on the image displaydevice 26. Accordingly, the operator looks at this state, and can judgea reliability of the axis alignment processing.

Embodiment 6

FIG. 13 is a flowchart for explaining a sixth embodiment of the presentinvention, which is an outline of the processing flowchart of anautomatic astigmatism alignment after executing an automatic axisalignment. The processing is divided into three groups of steps (firststep: Steps 6001 to 6003, second step: Steps 6004 to 6006, and thirdstep: Steps 6007 to 6009). In the first step, a correct focus positionof an objective lens is set. In the second step, the optimum value of anastigmatism corrector (X-direction) is set. In the third step, theoptimum value of an astigmatism corrector (Y-direction) is set.

In Step 6001, an initial value R0 of a control value of the objectivelens is determined by the present value R and a decided width ΔR. Theinitial value is obtained by R0=R−ΔR/2. In Step 6002, an image isfetched in while continuously increasing the control value of theobjective lens by a width dR, which is previously determined, from theinitial value R0. The width dR is obtained by dR=ΔR/N, where N is thenumber of evaluation points. The fetched-in image is transferred to animage processing device 27, and an evaluation value is calculated. Withrespect to the evaluation value, processings for obtaining a signalamount for four directions of 0°, 90°, 45° and 135° (for example, atotal sum of differentiation images) are executed in a range from R0 toR0+ΔR.

In Step 6003, based on the evaluation value calculated at N pointsrespectively in each of the four directions, an average of four controlvalues for obtaining the control value of the objective lens, which arebiggest in each direction, is set in the objective lens as the optimumvalue. In Step 6004, the initial value S0 of the control value of theastigmatism corrector (X-direction) is determined by the present value Sand the width ΔS previously determined. The initial value is obtained byS0=S−ΔS/2. In Step 6005, the image is fetched in while increasing thecontrol value from the initial value S0 thereof by the width dS. Thewidth dS is obtained by S=ΔS/N, and N is the number of evaluationpoints. The fetched image is transferred to the image processing device27, and the evaluation value is calculated. The evaluation value obtainsa signal amount of the whole of the image, for example, a total sum ofdifferentiation images. The above described processings are executedfrom S0 to S0+ΔS.

In Step 6006, among the evaluation values obtained by calculating for Npoints, the control value at which the evaluation value is maximum isset in the astigmatism corrector (X-direction) as the optimum value. InSteps 6007 to 6009, as to the astigmatism corrector (Y-direction), thesame processings as Steps 6004 to 6006 are executed.

According to this system, it is possible to detect the correct focusposition for the image including the astigmatism in the first step, andthe automatic astigmatism alignment can be executed by the image inwhich the correct focus position is set. On the contrary, when the firststep is executed after the second and third steps, the astigmatismalignment is executed by the image which does not exist in the correctfocus position, and it is difficult to obtain the optimum astigmatismcorrection value.

According to the present invention, it is possible to perform the axisalignment with high precision regardless of the optical condition of thecharged particle beam apparatus.

1. A charged particle beam alignment method which performs an axisalignment for a lens and uses a charged particle beam apparatus having alens for converging a charged particle beam emitted from a chargedparticle source and forming a sample image by detecting secondarycharged particles emitted from a sample by radiating the chargedparticle beam onto the sample converged by the lens, the methodcomprising the steps of: changing a convergence condition of saidobjective lens to two states when a deflection condition of saidalignment deflector is rendered to a first state; detecting a firstdeviation between first and second sample images obtained when thedeflection condition of said alignment deflector is rendered to thefirst state; changing the convergence condition of said objective lensto at least two states when the deflection condition of said alignmentdeflector is rendered to a second state; detecting a second deviationbetween third and fourth sample images obtained when the deflectioncondition of said alignment deflector is rendered to the second state;calculating an unknown changing depending on an operation condition ofsaid charged particle beam optical system by applying information of thefirst and second deviations to an equation finding the deviation of thesample image relative to a change of an alignment condition; andobtaining the alignment condition based on the calculated unknown and acondition in which an image deviation becomes small when the convergencecondition of the objective lens is changed to the two condition. 2.-23.(canceled)